(1) Field of the Invention
The invention relates to a display device. More specifically, the invention relates to a liquid crystal display device capable of increasing a display region relative to a predetermined outline region, that is, realizing a narrow frame through the photo-alignment process.
(2) Description of the Related Art
The liquid crystal display device is configured to include a TFT substrate in which pixels with pixel electrodes and thin film transistors (TFT) arranged in a matrix, and a counter substrate opposite the TFT substrate, having a color filter and the like formed at a location corresponding to the pixel electrode of the TFT substrate. A liquid crystal is interposed between the TFT substrate and the counter substrate. An image is formed by controlling light transmittance of the liquid crystal molecule for each pixel.
The liquid crystal display device with a flat and light structure has been widely used in various fields. A compact liquid crystal display device has been widely applied to the mobile phone and DSC (Digital Still Camera). It is strongly demanded that the compact liquid crystal display device should expand its display region while keeping the small outline region. In the aforementioned circumstances, the width defined by the end of the display region and the end of the liquid crystal display device is reduced, thus demanding the narrow frame.
A seal material for bonding the TFT substrate and the counter substrate is formed on the frame region. An alignment film for bringing the liquid crystal into an initial alignment state is formed on the display region of the liquid crystal display device. The alignment film is required to completely seal the display region, and accordingly, the area to which the alignment film is applied has to be larger than the display region by a predetermined width. Meanwhile, presence of the alignment film between the seal material and the TFT substrate or between the seal material and the counter substrate may impair bonding reliability of the seal material. The end of the applied alignment film, thus has to be strictly controlled.
The alignment process for the alignment film may be executed through any one of the rubbing process and the photo-alignment process. JP-A-2004-206091 discloses the use of photo-alignment to execute: (1) reduction in alignment disorder owing to complicated stepped structure of the pixel part, and (2) prevention of the influence of static electricity generated in rubbing, disordered tip of yarns of the rubbing cloth, and foreign substances generated by rubbing. The viewing angle is an important property for the liquid crystal display device. A method of IPS (In Plane Switching) rotates the liquid crystal molecules parallel to the substrate so as to control intensity of light that transmits through the liquid crystal layer, thus providing excellent viewing angle property. On the contrary, the liquid crystal display device of IPS type that requires no pre-tilt angle is suitable for the photo-alignment process.
The alignment film is applied through flexography or ink jetting. Upon application of the liquid material for forming the alignment film, it wettedly spreads. It is therefore difficult to control the end of the applied material. Especially when applying the alignment film through ink jetting, it is further difficult to control the application because of low viscosity of the material for forming the alignment film.
JP-A-2011-145535 discloses the structure that forms an additional frame-like alignment film outside the alignment film formed on the display region, which serves as the stopper for controlling the alignment film to be formed on the display region. The structure controls the application range of the alignment film on the display region.
FIG. 13 represents an example that an alignment film 102 is applied to an entire lower surface of a seal material 250. Referring to FIG. 13, an organic passivation film 101 is formed on a glass TFT substrate 100, on which the alignment film 102 is formed. Actually, films including a gate insulation film and an inter-layer insulation film are applied to the TFT substrate 100 in addition to the organic passivation film 101. Those films are not shown in FIG. 13. The organic passivation film 101 has its part eliminated to form a recess 104 in a region where a seal material 250 is formed. A TFT, a gate wiring, a video signal line and the like (not shown) are formed on the TFT substrate. It is necessary to prevent corrosion of metal used for forming those elements owing to moisture and the like. Inside of the seal portion will be invaded by external moisture through the organic passivation film 101. Such invasion of the external moisture is blocked by partially eliminating the organic passivation film 101. The alignment film 102 is formed on the organic passivation film 101.
Referring to FIG. 13, the alignment film 102 is formed on a glass counter substrate 200. Actually, films including the color filter, black matrix and overcoat film are formed on the counter substrate 200, which are omitted in FIG. 13 for easy understanding. The alignment film 102 is formed on those films. The counter substrate 200 and the TFT substrate 100 are bonded with the seal material 250.
The alignment films 102 formed on the TFT substrate 100 and the counter substrate 200 have been photo-alignment processed. The photo-alignment process includes photolysis type for dividing a specifically directed polyimide chain that constitutes the alignment film 102 through irradiation of polarized ultraviolet light with wavelength of 300 nm or shorter, photo-dimerization type (photo-crosslinking type) for crosslinking polymer that constitutes the alignment film 102 through irradiation of polarized ultraviolet light with wavelength ranging from 300 nm to 350 nm, photo-isomerization type for isomerizing the polymer that constitutes the alignment film 102 through irradiation of polarized ultraviolet light with wavelength of 350 nm or longer, and the type derived from combining any of the aforementioned types. Irradiation of polarized ultraviolet light through the process of arbitrary type as described above allows the alignment film 102 to exhibit uniaxial anisotropy.
FIG. 14 is a plan view illustrating a state where the seal material 250 is formed on the alignment film 102 at the side of the TFT substrate 100. The seal material 250 is formed on the same alignment film 102 as the one formed on a display region 300. Bonding strength to the seal material 250 is essential for the photo-alignment processed alignment film 102. Reliability of the seal portion is especially the key to the liquid crystal display device as shown in FIGS. 13 and 14 on the ground that the polymer on the surface of the alignment film 102 suffers deterioration by the photochemical reaction in the photo-alignment process, lowering the bonding strength between the alignment film and the seal material.
FIG. 15 is a sectional view of the structure for accurately controlling the outline of the alignment film 102 on the display region with another alignment film 106 which is formed around the alignment film 102 on the display region 300, and thicker than the alignment film 102 on the display region for preventing the aforementioned problem. The structure shown in FIG. 15 is disclosed in JP-A-2011-145535. Referring to FIG. 15, the alignment film 106 is formed to be partially overlapped with the seal material 250. The alignment film 106 is thicker than the alignment film 102 formed on the display region, which serves as the stopper for the alignment film 102 formed on the display region. As the alignment film 106 is overlapped with the seal material 250 only partially, deterioration in the bonding strength of the seal material 250 is substantially negligible.
As FIG. 15 illustrates, the alignment film 106 serves as the stopper for the alignment film on the display region. As the alignment film 102 to be formed on the display region approaches the alignment film 106, its thickness becomes large. The liquid crystal display device of IPS type is configured that a first electrode is formed to have a plane surface, an inter-layer insulation film is formed on the first electrode, and a second electrode with a plurality of slits is formed on the inter-layer insulation film so as to generate an electric field in the liquid crystal layer through the slits of the second electrode upon application of voltage between the first and the second electrodes, thus controlling rotation of the liquid crystal molecules under the electric field. Intensity of the electric field generated in the liquid crystal layer varies with the film thickness of the alignment film 102. This may cause uneven brightness especially around the display screen. However, an effort to form the display region only in the area where the change in the film thickness of the alignment film 102 is small may enlarge the frame region.